1. Field of the Invention
The present invention is related to the field of digital image processing, and more specifically to filtering to remove ringing artifacts from decompressed images.
2. Description of the Related Art
Images are stored in arrays of pixels. Each pixel stores a small portion of an image, in the form of image data. The same is true for both static images (such as photographs), and for video sequences.
The large volume of image data slows down its transmission. To alleviate the problem, compression techniques are used to reduce the amount of data. Sometimes high compression is used for fastest transmission.
A problem arises when a high compression technique is used with insufficient bit rate of transmission. When the image is reconstructed, it includes visual noise, which is also called ringing artifacts. These artifacts are introduced in the places of the image where high spatial frequency DCT (Discrete Cosine transform) components are substantially reduced. This occurs also around the dominant edges of the image.
The prior art has addressed this problem by applying a de-ringing filter, to remove the artifacts. This approach does not work satisfactorily. Either the artifacts are not removed fully, or the dominant edges are filtered too much. In that case, the dominant edges become so smoothened that they no longer appear as dominant, which detracts from the overall image quality.